The present invention relates to a method of producing radiation sensors, in particular for infrared radiation, with an absorber for the radiation to be measured and a plurality of thermoelements for measuring the absorbed radiation-induced heating of the radiation absorber integrated into a semiconductor substrate. These methods can be used in particular for production of radiation absorbers with a plurality of sensor elements integrated at small distances on a substrate.
Such a radiation sensor with a plurality of sensor elements and a method of producing same are described, for example, by I. H. Choi and K. D. Wise, xe2x80x9cA Linear Thermopile Infrared Detector Array with On-Chip Multiplexing,xe2x80x9d IEEE Trans Electron. Devices (September 1985), pages 132 through 135. This article describes a method whereby boron is diffused into a  less than 100 greater than  oriented silicon substrate in a ring pattern from the front side, a membrane of SiO2 and Si3N4 is created on the front side, and then openings are cut through the substrate from the rear side of the substrate by anisotropic wet etching. These openings end on the front side of the substrate within the ring-shaped areas doped with boron. This forms openings in the substrate that are closed only by the thin membrane. A radiation absorber is formed on the membrane in each of these openings. A plurality of thermoelements connected in series each has a hot contact in the vicinity of the radiation absorber and a cold contact on the remaining silicon substrate which functions as a heat sink.
This known manufacturing method has a number of disadvantages. The anisotropy of wet etching occurs due to the fact that the etching process takes place at different rates on the different crystal faces of the silicon substrate. The etching rate is lowest on a surface with  less than 111 greater than  orientation. Therefore, in wet etching of a  less than 100 greater than surface through an opening in a mask, a recess is formed in the surface, its side walls having  less than 111 greater than orientations and being inclined at an angle of approx. 54xc2x0 to the  less than 100 greater than  surface. The bottom surface of the resulting recess is smaller the further the etching operation proceeds into the material, until it reaches a depth where the opposite walls of the recess abut against one another. Therefore, to produce a small opening at the level of the membrane, a mask with a much larger opening must be formed on the opposite side of the substrate.
Fluctuations in thickness between different substrates or within a substrate have a critical effect on the dimensions of the opening produced in the membrane due to the inclined orientation of the walls. It is extremely difficult to produce precision openings with small dimensions in relation to the thickness of the substrate, because fluctuations in the thickness of the substrate have a great influence on their size.
This problem is counteracted in the cited literature by the diffused ring made of boron. The boron-doped material is not attacked by etching, so the opening in the mask on the rear side of the substrate can be larger than would be necessary in view of the crystal geometry of the substrate to obtain a given opening size in the membrane. The size of the finished opening is then determined by the diameter of the undoped region inside the boron-doped ring. However, one unavoidable consequence of this method is that a portion of the rear side of the boron-doped ring which has diffused into the substrate is exposed so that the thickness of the substrate in the immediate vicinity of the opening after etching is determined by the thickness of the ring, which amounts to only approx. 20 xcexcm. Although a greater ring thickness could be achieved, this would be possible only through long diffusion times at very high process temperatures. This leads to the problem that, in the finished infrared sensor, the boron-doped ring may be eroded to varying extents by the etching process, resulting in variations in the quality of heat transfer from the cold contacts of the thermoelement over the ring into the solid silicon substrate, which can lead to systematic measurement errors.
Another radiation sensor with a silicon substrate, a radiation absorber arranged on a membrane over an opening in the substrate and a plurality of thermoelements with a hot contact in the vicinity of the radiation absorber and a cold contact on the silicon substrate is known from German Published Patent Application No. 41 02 524. With this sensor, the walls of the opening also diverge toward the side of the substrate facing away from the membrane in the manner characteristic of anisotropic wet etching. The diameter of the opening is much greater than the thickness of the substrate.
The present invention provides methods of producing radiation sensors which make it possible to produce radiation sensors with precisely reproducible properties and permit the production of radiation sensors with a plurality of individual sensor elements which can be arranged at a small distance from one another, which is independent of the thickness of the substrate used.
According to a first aspect of the present invention, these advantages are achieved by the steps of forming an opening in the membrane in the specified area and etching the semiconductor substrate through this opening. This opening makes it possible to produce the required cavity below the radiation absorber from the front side of the substrate, thus eliminating the necessity of etching through the entire substrate in a time-consuming process. This eliminates possible sources of error in positioning the etching mask on the rear side of the substrate in relation to the position of the radiation absorber, as would otherwise be necessary; there is no danger of extensive etching beneath the edge areas of the opening where cold contacts of the thermoelement can be mounted, which would thus result in poor thermal contact with the solid silicon substrate; furthermore, there may be solid unetched semiconductor material a short distance below the radiation absorber, which increases the total mass of the heat sink formed by the semiconductor material.
According to a first variant of this method, wherever a recess is to be created, the semiconductor material is made porous in that area prior to deposition. This can be accomplished by an anodic oxidation, e.g., with an HF electrolyte, in an electrochemical process in which the wafer functions as the anode with respect to the electrolyte. This region, which has been made porous, can then be etched out selectively in a subsequent etching step.
This etching step preferably takes place after the membrane has been deposited and the thermoelements have been structured on the membrane.
To determine the area to be etched out, in this case the surface of the semiconductor substrate is preferably masked with a protective layer made of a material which is resistant to the agent used to make the semiconductor porous. This material may be chromium or gold, for example.
As an alternative, the areas to be etched out can be determined by low n-type doping (nxe2x88x92 doping) of the areas that are not to be etched away, so that in contrast with the p-doped substrate and any n++ doped areas, they are not attacked by the agent used to make the semiconductor porous. The etching step which follows the step of making the semiconductor porous may be a traditional wet etching step.
The thermoelements are preferably structured on the deposited membrane before the etching step.
No special masking is necessary for the etching step if the material to be etched out has been prepared by making it porous.
According to a second variant of this method, no preparation of the area to be etched out by making it porous is necessary, and instead the area to be etched out is determined only by the formation of the opening in the membrane. The recess can be produced easily by isotropic etching of the substrate area behind the opening.
This isotropic etching can be performed by electrochemical anodizing followed by dissolving, by direct electrochemical dissolution or by isotropic wet etching, e.g. HNA (HF+NHO3+CH3COOH).
In this second variant, however, dry etching methods are preferred, such as plasma etching or spontaneous dry etching, because gaseous etching media can penetrate more easily than liquid media into the area to be etched away behind the opening, and because the mass exchange through the opening is more effective.
In particular, plasmas of F2 in Ar, SF6 or NF3 may be used for plasma etching.
Gases such as XeF2, CIF3 or BrF3 which erode silicon immediately on coming in contact with it in a violent reaction, forming volatile SiF4, may be used for spontaneous dry etching.
According to a second aspect of the present invention, a method is to be provided of producing a radiation sensor on a semiconductor substrate by performing the steps of establishing at least one area on a first surface of the substrate, in which an opening is to be created in the substrate, depositing a membrane on the first surface of the substrate, applying a radiation absorber to the membrane, applying thermoelements with a hot contact in thermal contact with the radiation absorber and a cold contact in thermal contact with the semiconductor substrate, and applying a mask to a second surface of the semiconductor substrate (the rear side) which is opposite the first surface. According to this method, the mask has an opening congruent with each area thus determined, and the second surface is treated by anisotropic dry etching until the silicon substrate has been etched away in the areas determined.
Suitable anisotropic dry etching methods are described in German Published Patent Application No. 42 41 045. The plasma etching processes described there make it possible to produce openings in the semiconductor substrate with walls almost perpendicular to its surface, thus eliminating the necessity for making the openings in the etching mask on the second surface of the substrate much larger than the finished openings by including a lead produced later in the substrate at the level of the membrane. Not only does this eliminate possible sources of error in producing and positioning the mask, it also yields the possibility of arranging the individual openings at a much smaller distance from one another which no longer necessarily depends on the thickness of the substrate. The cross-sectional dimensions of the openings in the semiconductor substrate are constant over the thickness of the substrate, so that there are no losses of surface area and a dense packing of openings is possible.
It is readily possible to produce an opening in a substrate up to 700 xcexcm thick merely by anisotropic dry etching as described in the aforementioned publication.
According to two advantageous embodiments of the method, however, the openings are produced in two steps.
In the first embodiment, first a contiguous area of the substrate is eroded by etching, thereby producing the second surface on which the actual etching mask is applied. Any desired etching method, preferably a rapid etching method, can be used for this etching.
As an alternative, dry etching itself may be subdivided into two steps, with etching being performed only in the congruent openings in the mask in the first step and then performed on a contiguous area having several congruent openings. Due to the first step, the congruent openings have a certain xe2x80x9cprojectionxe2x80x9d over the contiguous area surrounding them, and this is preserved in the next step of etching the contiguous area.
The result of these two methods is a substrate with a contiguous recessed surface on the rear surface of the substrate and a plurality of openings extending from the surface to the membrane on the front surface of the substrate. The individual openings are separated by webs of semiconductor material, although the thickness of these webs is not that of the original substrate, but it is constant and reproducible for each individual opening, and thus for each individual radiation absorber arranged in such an opening it forms a uniform thermal tie to the heat sink formed by the semiconductor substrate.
To form an effective heat sink, the webs of semiconductor material should be at least 50 xcexcm thick. Etching should thus proceed at least to this depth in the congruent openings. At a high aspect ratio of the openings, it becomes more and more difficult for the material etched away to be removed from the openings, and furthermore, on reaching the membrane, there is the risk of notching at the interface between the semiconductor substrate and the membrane, so it may be expedient to limit the depth of etching in the congruent openings, e.g., to a value of 100 or 200 xcexcm.
A two-layer mask is preferably used for two-step dry etching, with an opening that corresponds to the contiguous area being formed in a first masking layer and with the second layer containing the congruent openings. In the transition between the two etching layers, only the second layer of the mask need be removed selectively. For example, an advantageous combination would be to use photoresist masking and SiO2 as a hard mask or a nitride oxide layer system such as that described in German Published Patent Application No. 41 29 206.